Dislocation removal by a last pass starting at a location displaced from the original seed into the grown crystal



May 26, 1964 SPORRER 3,134,700

DISLQCATION REMOVAL BY A LAST PASS STARTING AT A LOCATION DISPLACED FROMTHE ORIGINAL SEED INTO THE GROWN CRYSTAL Filed April 20. 1960 Fig. 1 2

United States Patent 3 134 700 DISLQQATKON. naMovALnY A LAST PASS START-ner. AT A LOCATION nrsrrncnn rRoM THE onronsnr. snnn INTO rrrs onownCRYSTAL Ludwig Sporrer, Erlangen, Germany, assignor to Siemens- Myinvention relates to the production of monocrystalline semiconductormaterial, such as silicon and germanium, for example, for electronicpurposes by floating Zone melting. Such a method, as a rule, is carriedout by fusing a monocrystalline seed to one end of a polycrystalline rodof the same semiconductor material. The rod is then subjected to zonemelting by heating a narrow cross-sectional zone of the rod and movingthe molten zone longitudinally along the rod. When the zone meltingoperation is started at the junction point of the seed crystal and themolten zone is passed through the rod, the semiconductor materialrecrystallizes out of the molten zone'in monocrystalline condition. Byrepeating; the zone passes, a hyper-pure monocrystalline rod can beobtained. The seed crystal can thereafter be severed from the rod andcan be used for similar processing ofv other polycrystalline rods.

In the known method, the seed crystal can be used for processing only alimited number of semiconductor rods because further use of the sameseed crystal would greatly impair the electric properties of theadditionally processed rods. This is because when the rod is heated upat the rod end adjacent to the crystal seed prior to each individualzone pass, the temperature of the then stationary heated and molten zonemay cause crystal-lattice dislocations which, with an increasing numberof zone passes, migrate from the end of the seed into a portion of therod being processed. Such lattice dislocations can be mademicroscopically visible by etching of the crystal surface, and thenmanifest themselves by etch pits. It has been found that semiconductorrods thus converted from polycrystalline to monocrystalline constitutionexhibit a greatly decreased lifetime of the minority charge carriers inthe rod portion that was adjacent to the seed during processing. This isa serious defect because the lifetime of the charge carriers insemiconductor rods for the production of electronic semiconductorelements or wafers must be as high as possible and should be uniformover the entire length of the rod.

The mentioned detrimental effect can be minimized by replacing thecrystal seed by a new seed prior to performing the ultimate pass of zonemelting operation. This, however, is extremely intricate, particularlywhen performing the zone melting process invacuum.

It is an object of my invention to avoid the abovementioned deficienciesin a considerable simpler, more economical, and less time consumingmanner.

Relating to a zone melting operation for the production of amonocrystalline, hyper-pure semiconductor rod for electronic purposesfrom a polycrystalline rod by crucible-free zone melting with the aid ofa crystal seed, wherein the molten zone is repeatedly passed in a givendirection lengthwise through the semiconductor rod, each time commencingfrom the fusion junction of the seed, it is a feature of my inventionthat the commencement ofthe ultimate pass of zone melting is shiftedfrom the fusion junction of the seed away to a location of the rod thatpossesses a lesser degree of crystal lattice dislocations than obtainingnear the junction.

For further explaining the invention, reference will be had to theaccompanying drawing comprising two figures,

ICCv

each showing a portion of a zone melting device, in front view andpartly in section.

Although the methods illustrated in FIGS. 1 and 2 have certain featuresin common, they will be individually described below.

Referring to FIG. 1 a semiconductor rod 11 of poly-. crystallinesemiconductor material, such as silicon, is coaxially fused togetherwith a longitudinally aligned monocrystalline seed 12, the (lll)-crystalaxis of which coincides with the travelling direction of the molten zone14, which generally has a mass supportable by adhesion to the adjacentsolid ends. The narrow floating zone 14 of the rod 12 is melted in ahigh-frequency alternating field produced by a water-cooledhigh-frequency coil 13. The turns of coil 13. consist of silver tubingand are traversed by cooling water supplied through bores of the currentsupply terminals 15 to which the coil 13 is mechanically andelectrically attached. The coil 13 is displaceable in the longitudinaldirection of the rod 11 to permit the passing of the molten zone 14lengthwise through the rod. The crystal seed 12 is fastened in a holdingand centering device 17 firmly joined by means of a clamping screw 18.with a drive shaft 19 which permits maintaining the seed 12 and thelower portion of the rod 11 in rotation during progress of the zonemelting operation. The upper end (not illustrated) of the rod 11 islikewise fastened to a centering holder and may be likewise rotated asindicated by the arrow.

As a rule, a given number of passes, for example ten, are required forcompletely purifying the polycrystalline rod and converting it into amonocrystal. Each individual pass is commenced at the end of the crystalseed 12, this location being identified by a dot-and-dash line 20. Ninezone passes, all starting at line 20 and progressing to the upper rodend, are then completed. However, the tenth and last pass of the zonemelting operation is commenced at a somewhat higher location 21 of therod, which location may be spaced from line 20 a distance approximatelyequal to the diameter of the rod 11, and is located above the end of thecrystal seed. At location 21, the completely processed rod is sawed offthe seed end, and the rodnext to be processed is fused to the crystalseed. Consequently, after processing of the first rod, the seed crystalis extended by a piece denoted by 22. A piece 23 of the same length canbe out from the lower end of the seed if necessary.

With the above method, each subsequent polycrystal-v line rod to beprocessed is always fused together with a crystal seed at a seed endconsisting of hyper-pure monocrystalline material. As a result, thesemiconductor rods processed have uniformly good electric propertiesregardless of the number of times one and the same seed is used.

The method has the further advantage that the molten zone does not passthrough a locality where the cross section of the rod changesappreciably. This can be utilized incases where; the zone meltingequipment is automated and the zone melting operation is controlled independence upon the electric current passing through the heater coil.The zone melting operation can then be regulated for constancy of, thejust mentioned current.

For the production of extremely pure monocrystalline, semiconductor rodsfrom. undoped semiconductor material up to about 20 zone passageslegnthwise through the rod may be required. It has been found that thegreat number of zone passages may result in impairing the lifetime ofthe minority charge carriers because the recombination possibilitybecomes increased by crystal dislocations. Such dislocations occur inthe seed crystal when fusing it together with the polycrystalline rod.The result of such dislocations is to unfavorably modify the crystalstructure at the end of the seed down to approximately 2 cm. into theseed body. During the subsequent zone melting starting from the fusionbond, crystal displacements are dragged from the end of the seed intothe rod being processed. The dislocations thus caused to migrate intothe rod increase with an increasing number of zone passages and thuspenetrate more and more into the semiconductor rod. It is a furtherobject of my invention to eliminate such shortcomings and to thus renderthe production method more economical by reducing the quantity ofunsuitable products. This object is achieved by substantiallyeliminating the crystal dislocations resulting from the fusionattachment of the seed to the rod at an earliest possible processingstep. According to a more specific feature of the invention, thecommencement of one of the first few zone passages effected after theseed is fused together with the polycrystalline rod, is located awayfrom the fusion bond into the seed crystal. Preferably, this is done notlater than in the fourth zone passage. This aspect of the invention willbe further explained with reference to FIG. 2.

A monocrystalline seed 111 coaxially fused together with apolycrystalline semiconductor rod 121 to be converted to monocrystallineconstitution is held in a holder comprising a cylindrical member 141 ofceramic material, a centering cup 151, a member of molybdenum strips 13uniformly distributed about the periphery of the seed and acting assprings. The ceramic member 141 is held in proper position by a numberof set screws in threaded engagement with the cup 151. The cup, andhence the seed 111, can be placed in rotation by means of a shaft 171fastened by a screw bolt 161 to the centering cup 151. The upper portionof the rod 121 and its upper holder are not illustrated. A heatingdevice permits melting a narrow zone 201 of the rod 111. The heatercomprises an inductance coil 181 consisting preferably of copper tubingwhich, during operation, is traversed by cooling liquid. The coil 181has its ends fastened to holders 191 which operates as current supplyterminals for high-frequency alternating current.

During operation of the device the heater winding 181 is gradually movedupward from the seed 111 toward the opposite end of the rod, and thisprocess is repeated. If for the production of a hyper-puremonocrystalline rod of semiconductor material, for example silicon, apredetermined number, for example 20, zone passages are necessary, thenthe first zone passage is preferably commenced immediately after fusingthe seed 111 together with the rod 121 at the fusion point indicated bya dotand-dash line 211. The next following zone passage is thencommenced within the seed crystal at the location denoted by 221, sothat the floating melting zone is pulled through the fusion point 211.The space of location 221 from the fusion bond 211 is preferably made 2cm. or more. The next following zone passages again commence at thelocation 211. The commencement of the last zone passage can be placed ata location 231 away from the fusion bond into the rod being processed,thus being spaced from the fusion point 211 where crystal dislocationscan be caused by the repeated melting. The location 231 is so chosenthat the crystal structure is practically not affected and exhibits agreater degree of perfection.

Aside from improving the lifetime of the minority carrier in the rodportion adjacent to the seed, the methods of the invention have thefurther advantage that the enire process of converting a polycrystallinerod into a hyper-pure monocrystal can be completely performedautomatically with automatic devices regulating the performance forconstant current.

Iclaim:

1. A crucible-free, floating molten zone method of making amonocrystalline silicon semiconductor solid body from a polycrystallinesilicon body, comprising supporting the polycrystalline solid bodyvertically, fusing a monocrystalline silicon seed crystal to thepolycrystalline silicon body, heating a transverse zone to produce amolten zone extending over the cross section of the body, repeatedlypassing said molten zone along said initially polycrystalline body inone and the same direction, the passes starting at a point to includethe location of fusion of the seed crystal to the polycrystalline body,the passes converting polycrystalline to monocrystalline silicon, andthereafter passing said molten zone in said direction but starting atanother location removed from said location of fusion of the seedcrystal but within the originally polycrystalline silicon body which isnow monocrystalline silicon.

2. A crucible-free floating molten zone method of making amonocrystalline semiconductor body from a polycrystalline body,comprising supporting the polycrystalline body vertically, fusing amonocrystalline seed crystal to the polycrystalline body, heating atransverse zone to produce a molten zone extending over the crosssection of the body, repeatedly passing said molten zone along saidinitially polycrystalline body in one and the same direction, the passesstarting at a point to include the location of fusion of the seedcrystal to the polycrystalline body, the passes convertingpolycrystalline to monocrystalline, and thereafter passing said moltenzone in said direction but starting at another location removed, atleast a distance equal to the diameter of the rod, from said location offusion of the seed crystal but within the originally polycrystallinebody which is now monocrystalline.

3. A crucible-free, floating molten zone method of making amonocrystalline germanium semiconductor body from a polycrystallinegermanium body, comprising supporting the polycrystalline bodyvertically, fusing a monocrystalline germanium seed crystal to thepolycrystalline germanium body, heating a transverse zone to produce amolten zone extending over the cross section of the body, repeatedlypassing said molten zone along said initially polycrystalline body inone and the same direction, the passes starting at a point to includethe location of fusion of the seed crystal to the polycrystalline body,the passes converting polycrystalline to monocrystalline germanium, andthereafter passing said molten zone in said direction but starting atanother location removed from said location of fusion of the seedcrystal but within the originally polycrystalline germanium body whichis now monocrystalline germanium, severing the so producedmonocrystalline rod at a location that is outside of the seed crystaland is within the originally polycrystalline rod, but which is nowmonocrystalline, removing the severed monocrystalline rod portion soproduced, fusing a second polycrystalline rod to the other portion, atthe location of severing, and repeating the passes defined above.

4. A crucible-free, floating molten zone method of making amonocrystalline silicon semiconductor body from a polycrystallinesilicon body, comprising supportmg the polycrystalline body vertically,fusing a monocrystalline silicon seed crystal to the polycrystallinesilicon body, heating a transverse zone to produce a molten zoneextending over the cross section of the body, repeatedly passing saidmolten zone along said initially polycrystalline body in one and thesame direction, the passes starting at the location of fusion of theseed crystal to the polycrystalline body, the passes convertingpolycrystalline silicon to monocrystalline silicon, and thereafterpassing said molten zone in said direction but starting at anotherlocation removed from said location of fusion of the seed crystal butwithin the originally polycrystalline silicon body which is nowmonocrystalline silicon, severing the so produced monocrystalline bodyat a location therein that is outside of the seed crystal and is withinthe originally polycrystalline silicon rod, but which is nowmonocrystalline silicon, and is removed from said location of fusion ofthe seed crystall, removing the severed monocrystalline body portion soproduced, fusing a second polycrystalline silicon body to the otherportion, at the location of severing, and repeating the passes definedabove.

5. A crucible-free, floating molten zone method of making amonocrystalline silicon semiconductor rod from a polycrystalline siliconrod, comprising supporting the polycrystalline rod vertically, fusing amonocrystalline silicon seed crystal to the polycrystalline silicon rod,heating a transverse zone to produce a molten zone extending over thecross section of the rod, repeatedly passing said molten zone along saidinitially polycrystalline rod in one and the same direction, the passesstarting at a point to include the location of fusion of the seedcrystal to the polycrystalline rod, the passes converting thepolycrystalline silicon rod to a monocrystalline silicon rod, andthereafter passing said molten zone in said direction but starting atanother location removed from said location of fusion of the seedcrystal but within the originally polycrystalline silicon rod which isnow monocrystalline silicon, severing the so produced monocrystallinebody at a location therein that is outside of the seed crystal and isWithin the originally polycrystalline silicon rod, but which is nowmonocrystalline silicon, and is removed from said location of fusion ofthe seed crystal at least a distance substantially equal to the diameterof the rod being processed, removing the severed monocrystalline rodportion so produced, fusing a second polycrystalline silicon rod to theother portion, at the location of severing, and repeating the passesdefined above.

6. A crucible-free, floating molten zone method of making amonocrystalline semiconductor rod from a polycrystalline rod, comprisingsupporting the polycrystalline rod vertically, fusing a monocrystallineseed crystal to the polycrystalline rod, heating a transverse zone toproduce a molten zone extending over the cross section of the rod,repeatedly passing said molten zone along said initially polycrystallinerod in one and the same direction, the passes starting at a point toinclude the location of fusion of the seed crystal to thepolycrystalline rod, the passes converting the polycrystalline rod to amonocrystalline rod, and thereafter passing said molten zone in saiddirection but starting an another location removed, at least a distanceequal to the diameter of the rod, from said location of fusion of theseed crystal but within the orginally polycrystalline rod but is nowmonocrystalline, severing the so produced monocrystalline body at alocation therein that is outside of the seed crystal and is within theoriginally polycrystalline silicon rod, but which is now monocrystallinesilicon, and is removed from said location of fusion of the seed crystalat least a distance substantially equal to the diameter of the rod beingprocessed, removing the severed monocrystalline rod portion so produced,fusing a second polycrystalline rod to the other portion, at thelocation of severing, and repeating the passes defined above.

7. The method defined in claim 6, the semiconductor being germanium.

8. A method for producing a monocyrstalline hyperpure semiconductor rodfor electric purposes from a polycrystalline rod by crucible-freefloating zone melting with the aid of a monocrystalline seed crystalfused to the rod by multiple zone passes in a common direction,comprising supporting the rod vertically, repeatedly passing the moltenzone lengthwise through the semiconductor rod commencing each time atthe fusion bond of the seed crystal, the method including the step ofcommencing at least one of the zone passes following the fusion of theseed to the rod at a location away from the fusion point into the seedcrystal, the commencement of the last zone passage being placed awayfrom the fusion point into the rod being processed at a distance atleast equal to the rod diameter, severing the so producedmonocrystalline body at a location therein that is outside of the seedcrystal and is within the originally polycrystalline silicon rod whichis now monocrystalline silicon, and is removed from said location offusion of the seed crystal as least a distance substantially equal tothe diameter of the rod being processed, removing the severedmoncrystalline rod portion so produced, fusing a second polycrystallinesilicon rod to the other portion, at the location of severing, andrepeating the passes defined above.

9. The method defined in claim 8, the seed crystal and the rod beingsilicon.

10. A method for producing a monocrystalline hyperpure semiconductor rodfor electronic purposes from a polycrystalline rod by crucible-freefloating zone melting with the aid of a seed crystal fused to the rod,comprising supporting the polycrystalline rod vertically; repeatedlypassing the molten zone lengthwise through the semiconductor rodcommencing each time at a point to include the location of the fusionjunction of the seed crystal to the rod, then commencing a zone meltingpass at a location in the said rod, namely outside of the seed crystal,and away from the fusion point of the seed crystal, the said location ofthe rod having lesser crystal lattice dislocations than said fusionjunction, severing the so produced monocrystalline rod at a locationthat is outside of the seed crystal and is within the originallypolycrystalline rod which is now monocrystalline, removing the severedmonocrystalline rod portion so produced, fusing a second polycrystallinerod to the other portion, at the location of severing, and repeating thepasses defined above.

vl1. A crucible-free, floating zone method for producing amonocrystalline hyperpure semiconductor rod for electronic purposes froma polycrystalline material comprising supporting a rod of thepolycrystalline semiconductor material with its axis extendingvertically, heat fusing a seed crystal to an end of said rod,diminishing the crystal dislocations in the fusion zone, resulting fromthe fusion attachment of the seed to the rod, by commencing a moltenzone passage at a location displaced away from the fusion bond into theseed crystal a distance of at least two centimeters, said zone passagebeing then passed through the fusion bond into the rod, thereaftercarrying out several molten zone passages commencing from the fusionbond and passing into said rod to convert the rod into monocrystallinematerial, and thereafter making another zone passage commencing at alocation in said rod displaced away from the fusion zone a distance atleast equal to the diameter of the rod, where the crystal dislocationsare less than at the fusion bond.

12. A crucible-free, floating zone method for producing amonocrystalline hyperpure semiconductor rod for electronic purposes froma polycrystalline material, comprising supporting a rod ofpolycrystalline semiconductor material with its axis extendingvertically, fusing a seed crystal to an end of said rod, crystaldislocations being caused in the fusion bond by said fusion, thereaftercarrying out several molten zone passages commencing from the fusionbond and passing into said rod to convert the rod into monocrystallinematerial, and thereafter making another zone passage commencing at alocation in said rod displaced away from the fusion zone a distance atleast equal to the diameter of the rod, where the crystal dislocationsare less than at the fusion bond.

References Cited in the file of this patent UNITED STATES PATENTS2,768,914 Buchler et a1. Oct. 30, 1956 2,907,715 Cornelison Oct. 6, 19592,972,525 I Emeis Dec. 28, 1962 OTHER REFERENCES Keck et al.: PhysicsReview, vol. 89, March 15, 1953, page 1297 Crystallization of SiliconFrom a Floating Liquid Zone.

Keck et al.: Review of Scientific Instruments, vol. 25, No. 4, April1954, pages 331-334, Floating Zone Recrystallization of Silicon.

1. A CRUCIBLE-FREE, FLOATING MOLTEN ZONE METHOD OF MAKING AMONOCRYSTALLINE SILICON SEMICONDUCTOR SOLID BODY FROM A POLYCRYSTALLINESILICON BODY, COMPRISING SUPPORTING THE POLYCRYSTALLINE SOLID BODYVERTICALLY, FUSING A MONOCRYSTALLINE SILICON SEED CRYSTAL TO THEPOLYCRYSTALLINE SILICON BODY, HEATING A TRANSVERSE ZONE TO PRODUCE AMOLTEN ZONE EXTENDING OVER THE CROSS SECTION OF THE BODY, REPEATEDLYPASSING SAID MOLTEN ZONE ALONG SAID INITIALLY POLYCRYSTALLINE BODY INONE AND THE SAME DIRECTION, THE PASSES STARTING AT A POINT TO INCLUDETHE LOCATION OF FUSION OF THE SEED CRYSTAL TO THE POLYCRYSTALLINE BODY,THE PASSES CONVERTING POLYCRYSTALLINE TO MONOCRYSTALLINE SILICON, ANDTHEREAFTER PASSING SAID MOLTEN ZONE IN SAID DIRECTION BUT STARTING ATANOTHER LOCATION REMOVED FROM SAID LOCATION OF FUSION OF THE SEEDCRYSTAL BUT WITHIN THE ORIGINALLY POLYCRYSTALLNE SILICON BODY WHICH ISNOW MONOCRYSTALLINE SILICON.